vm_fontsize = 50

rem(px) 
  (px / vm_fontsize) * 1rem

vm_design = 750
html {
    font-size: (vm_fontsize / (vm_design / 2)) * 100vw; 
    /* // 同时，通过Media Queries 限制根元素最大最小值 */
    @media screen and (max-width: 320px) {
        font-size: 42.66px;
    }
    @media screen and (min-width: 540px) {
        font-size: 72px;
    }
}

body
  max-width 540px
  min-width 320px
  margin 0 auto
